1. Field of the Invention
This invention relates to a lyo gel, to its production and to its use for sealing, more particularly devices involving the use of electrical current. The lyo gel consists of at least two components, namely a network of a copolymer containing at least one comonomer bearing functional groups of carboxylic acid and carboxylic anhydride on the one hand and polyvalent metals on the other hand as gelling agent and an organic liquid as dispersant.
2. Discussion of Related Art
One such lyo gel is described in DE-OS 26 49 544, according to which a gel is formed from an alkyl acrylate copolymer of alkyl acrylate, .alpha.,.beta.-unsaturated carboxylic anhydrides, more particularly 0.2 to about 20% by weight maleic anhydride, and ethylenically unsaturated monomers, which is dissolved in an organic solvent, and from a metal alkoxide on storage at room temperature (see page 10, lines 4 to 8). The organic solvents are readily volatile substances so that the polymer solution rapidly dries (see page 12, paragraph 2). Since gelation is undesirable, at least 20% by weight of a lower aliphatic alcohol is added and the metal alkoxides are chelated (see page 5, last paragraph to page 6, line 3 and page 10 from line 12 to page 11, paragraph 3). Octylene glycol, triethanolamine, 2,4pentanedione or lactic acid are specifically mentioned, acetyl acetonates being preferred. The copolymer solution thus stabilized is used as an adhesive. After removal of the solvent, including the alcohol, the copolymer crosslinks and gives bonds of high cohesive strength (see page 13, paragraph 2).
A similar copolymer is described by Milka and Czech in "Adhasion" (1985), pages 29 to 32. A copolymer of 2-ethylhexyl acrylate, n-butyl acrylate and hydroxyethyl acrylate and also acrylic acid is dissolved in ethyl acetate, methyl ethyl ketone and acetone and crosslinked with titanic acid esters and metal acetyl acetonates (see page 29, middle column). Where organoyl titanate is used, a gel is formed, even in the presence of alcohols (see page 31, middle column). However, this is undesirable. Instead, the polymer solutions are used as contact adhesives because the acrylate copolymers crosslink spontaneously after evaporation of the alcohol and the other solvents (see page 29, left-hand column).
Other pressure-sensitive adhesive mixtures of the type in question are described in DE-OSS 24 16 991 and 23 37 558.
In all these cases, the object is to produce pressure-sensitive adhesives. The formation of gels is undesirable. The gels nevertheless obtained are attended, for example, by the disadvantage that, for a solids content of 10 to 60%, the rest of the solvent has to be removed which involves a contraction in volume (page 21 of DE-OS 24 16 991). A reduction in volume during drying also occurs in the case of DE-OS 23 37 558. The gels obtained are not fluid at room temperature and do not have an adequate sealing effect. There is no reference to their use as sealants.
The function of sealants is to fill gaps, joints and spaces between bodies in a volume-bridging, gas-tight and liquid-tight manner by plugging. Thus, spaces in cables and their connecting elements, such as free connectors, raceways, joint boxes, etc., are frequently filled to seal them against environmental influences, such as water, dust, air, oil, etc. and to provide mechanical and electrical protection.
Sealing systems based on the following macro-molecular substances are used for this purpose:
A) thermoplastic polymers, particularly hotmelt adhesives and injection molding compounds, PA1 B) crosslinking systems (thermosets), such as epoxides, polyurethanes, etc., PA1 C) crosslinking systems (elastomers), such as silicones and other rubbers, and PA1 D) waxes, bitumen, fats and oils. PA1 1.) The sealants shrink during curing and at temperatures between the application temperature and the in-use temperature. This gives rise to stresses which can even lead to cracks and leaks, particularly in the case of systems A), B) and D). PA1 2.) The sealants can only be used within a limited temperature range because, outside this more or less narrow range, they lose mechanical strength, break, flow out or decompose. This applies in particular to systems A), B) and D). PA1 3.) In some cases, the sealants show inadequate adhesion to various materials. This generally applies to systems A), B), C) and D). PA1 5.) The sealants show inadequate resistance to environmental influences, such as solvents, oils, water, etc. This applies to systems A) and D) and partly to B). PA1 6.) Hazardous components and aggressive decomposition products are formed during mixing, filling or curing. This applies to systems B) and C). PA1 1) should be transportable and flowable per se or under pressure within the processing time in order to fill spaces, PA1 2) should be elastically deformable under mechanical pressure within the elastic limits after gelling and PA1 3) should permanently exhibit any electrical properties required, such as dielectric constant, dielectric strength and volume resistivity.
The known systems have the following disadvantages:
4.) The sealants are not sufficiently plastic, for example when further contacts are introduced into the already filled space. This applies to all the sealing systems A), B), C) and D).
In "Adhesives Age" (1989), pages 24 to 29, Dillmann and Sanders describe adhesives and sealants based on a thermoplastic rubber of a block terpolymer containing maleic anhydride groups (Kraton FG 1901X). To produce a sealant, 100 parts by weight of this copolymer are mixed for 45 minutes at 80.degree. C. with 270 parts by weight Regalrez 1018 (an aliphatic hydrocarbon resin), 50 parts by weight Endex 160 (an aromatic hydrocarbon resin), 1 part by weight Irganox 1010 (a heat stabilizer), 1 part by weight Tinuvin 770 (a light absorber) and 1.5 parts by weight Tinuvin P (a UV absorber). The sealants obtained have melt viscosities of 0.7 to 2.6 Pa.multidot.s at 177.degree. C. Sealants such as these have the disadvantage that they lose their dimensional stability beyond 60.degree. C. In addition, they are attended by the disadvantages listed under points 1.), 2.), 4.) and 5.).
The problem addressed by the present invention was to avoid these disadvantages and, in addition, to provide a system which would be easy to handle. More particularly, the problem addressed by the present invention was to provide a sealant without any of the described disadvantages, particularly shrinkage, and with increased heat resistance. In addition, it would be useable over a wide temperature range and would be easy to handle.
According to the invention, this problem has been solved by a lyo gel in which the gelling agent is an organic fully synthetic polymer, which is crosslinked via its carboxylic acid groups and via metal compounds, and the swelling agent is a low-volatility, inert organic liquid having a volatility, as defined by its evaporation loss, of less than 0.5% by weight after 2 hours at 105.degree. C. in accordance with ASTM D 972. The extent of swelling and the degree of crosslinking are determined by the properties required. The starting mixture of polymer solution and the solution or suspension of the metal compound
The expert knows or can determine by conducting a few tests what the extent of swelling and the quantities of polymer components with and without carboxylic acid derivatives should be. A ratio by weight of swelling agent to gelling agent of 1 to 20:1 is preferred. The polymers preferably contain 1 to 5% by weight components containing a carboxylic acid or its derivative.
The polymers according to the invention are homopolymers or copolymers (both statistical polymers and block and graft polymers), such as polyurethane, polyamide, polyester and polymers of monomers containing olefinic double bonds. Preferred polymers are copolymers containing an olefinic double bond which contain maleic acid or derivatives thereof, such as the anhydride and ester, either grafted on or incorporated in the chain, more particularly thermoplastic elastomers, such as styrene/ethylene/butylene/styrene block polymers containing grafted acid groups. They are commercially obtainable or may be prepared by known methods. "Functional groups" are understood to be groups which react with the metal compounds under processing conditions. In addition to the carboxylic acids, carboxylic acid esters and carboxylic anhydride groups, OH and amine groups may also be present. Although reacting only slightly, if at all, on their own, these groups are active with the other functional groups present. The free carboxylic acids react much more quickly than the corresponding anhydrides. If the functional groups are in adjacent positions, for example in the case of maleic acid, a distinctly lower concentration is sufficient than where the groups are distributed purely statistically for obtaining adequate crosslinking under otherwise the same conditions. However, the carboxylic acid groups can also be positioned at the end of the chain. Suitable polymers are ELVAX 4320 (a copolymer of ethylene, an acid comonomer and vinyl acetate having an acid value of 4 to 8), LOTADER 8750 (a copolymer of ethylene, maleic anhydride and acrylic acid ester having an acid value of 17 and a melt index of 400), KRATON FG 1901 X (an SEBS block copolymer with maleic anhydride grafted on which may be hydrolyzed or reacted with alcohols, for example ethanol) and MACROMELT 6208 (a copolyamide having an acid value of &gt;5 and an amine value of &lt;0.8).
Metals which can be used for crosslinking are metals of the 2nd, 3rd, 4th and 5th main groups and also transition metals, more particularly ruthenium, vanadium, chromium, cobalt, manganese, aluminium, iron, titanium, zirconium, hafnium, tin, niobium, vanadium and cerium. However, divalent metals, such as copper titanyl, cadmium, cobalt, manganese, nickel, palladium and zinc are also effective. Preferred metals are iron and zirconium. The metals are used in the form of their complexes, particularly their chelates. Suitable chelating agents are, for example, octylene glycol, triethanolamine, acetyl acetone or lactic acid. Acetyl acetone is preferred. The chelates are prepared from the metal alkoxides and the chelating agent. Their concentration influences both the degree of crosslinking and the gelling time. A molar ratio of carboxylic acid or carboxylic acid ester on the one hand and metal compound on the other hand of 0.3 to 3:1 and, more particularly, 1 to 2:1 is preferred. The metal chelates are preferably added to the polymer solutions in the form of a suspension or solution.
The low-volatility inert solvents are liquids, such as natural oils, for example mineral oil and vegetable and animal oils, and synthetic oils, for example poly-a-olefins having the general formula R.sub.1 --CH.sub.2 (CH.sub.2 --CHR).sub.x --H, dicarboxylic acid esters having the general formula ROOC--(CH.sub.2).sub.x --COOR, diaryl alkanes or dialkyl benzenes, esters of polyols with carboxylic acids, polyglycols having the general formula R.sub.1 --O(CHR--CH.sub.2 --O--).sub.x --H, ortho phosphoric acid esters and silicone oils. Mixtures of several solvents may also be used. Preferred solvents are mineral oils and poly-a-olefins, dialkyl benzenes and diaryl alkanes. The solvents form a clear solution with the starting polymers. Their rheological behavior is comparable with that of a molecularly disperse solution. The solvent may also be used to suspend or dissolve the metal compound.
The gel may be present in the chelating agent in very small quantities.
The lyo gel according to the invention may contain other additives, although they should not affect its basic properties, more particularly fillers, diluents, stabilizers and oxidation inhibitors. The following are mentioned in particular: powder-form polymers, solid beads and hollow beads of glass or plastics, conductivity black, carbon fibers, silica, silicate and magnetic powders.
The lyo gel has the following remarkable properties: there is virtually no change in its volume compared with the starting mixture. It is dimensionally stable and elastic over a wide temperature range (-40.degree. to more than 200.degree. C.) without melting. It shows good adhesion to metals, such as copper and aluminium, and to plastics, such as polyethylene, polypropylene and nylon. It is readily and extremely formable over wide ranges, the ranges themselves being variable through the ratio of polymer network to organic dispersant and through the degree of crosslinking. It is largely chemically resistant. During the reaction, only the chelating agents are eliminated. The chelating agents are preferably alcohols and ketones, i.e. not aggressive or hazardous decomposition products.
The lyo gel according to the invention is generally produced as follows: 100 parts by weight of the polymer containing 5 to 100 g functional groups per kg polymer are dissolved in 50 to 2,000 parts by weight of the low-volatility inert swelling agent. The mixture is then heated in the usual way, for example to 100.degree. to 150.degree. C., stirred and then cooled. The preferably finely ground metal chelates are suspended or dissolved in the inert organic swelling agent with low vapor pressure (ratio by weight 1:1 to 10). They may be prepared from alcoholates by addition of the chelating agents.
100 Parts by weight of the polymer solution are mixed with 0.1 to 5 parts by weight of the metal compound in powder form, but preferably in the form of a suspension or solution, at room temperature in order to obtain uniform distribution. The mixture reacts in about 1 minute to 1 day (depending on its formulation) at room temperature and is liquid and transportable for that period, so that it can easily be introduced into a mold free from any bubbles. The reaction temperature is not critical. The reaction may take place both at -30.degree. C. and at +60.degree. C. The dimensionally stable lyo gel is obtained after the set gelling time of up to 1 hour.
The following reaction conditions are preferred: the following solvents are used for the preferred polymers, namely styrene/ethylene/butadiene/styrene copolymers and other copolymers of comonomer with an olefinic double bond containing maleic acid and derivatives thereof, such as anhydride and ester, either grafted on or incorporated in the chain: mineral oils, dialkyl benzenes, diaryl alkanes and poly-.alpha.-olefins.
The metal compounds have a particle diameter of preferably less than 50 .mu.m. They are suspended or dissolved in the solvent of the polymer or in another suitable liquid. However, the metal compound may also be dissolved with addition of a relatively high-boiling hydroxy compound, for example in benzyl alcohol or in NECIRES EPX-L (a thermoplastic resin based on aromatic vinyl compounds and copolymerized phenols, OH value 65, a product of Nevcin Polymers B.V., Holland). However, the relatively high-boiling hydroxy compound may also be added to the solution of the polymer. The reaction preferably takes place at room temperature.
By virtue of its positive properties and its simple production, the lyo gel is particularly suitable for sealing devices involving the use of electrical current because the initially two-component reaction solution is sufficiently liquid at room temperature to fill even small spaces, such as gaps and joints between bodies. Its production is easy to control. This applies both in regard to the temperature, the evolution of heat and to the gelling time and in regard to non-critical mixing ratios. The plugging effect obtained is particularly good and ageing-resistant over wide temperature ranges by virtue of the firm adhesion to metals and plastics and the elastic properties. Accordingly, the space is effectively filled in gas-tight, liquid-tight and dust-tight manner. The gel is particularly effective against water. Accordingly, the gel according to the invention is particularly suitable for filling spaces in cables and their connecting elements, such as free connectors, raceways, joint boxes, etc.